1. Field of the Invention
The present invention relates to a photovoltaic device to be used preferably for solar cell, photosensors, solid state pickup devices, etc., and, more particularly, to a stack type photovoltaic device with good energy conversion efficiency.
2. Related Background Art
A variety of machinery and instruments employ photovoltaic devices including solar cells as a driving energy source and photosensors as a light receiving device.
Photovoltaic devices such as solar cells have p-n or p-i-n junctions and generally use silicon as a semiconductor for the p-n or p-i-n junctions. Single crystal silicon is preferred from the viewpoint of conversion efficiency from light energy to electromotive force, while amorphous silicon is advantageous from the viewpoint of large area formation and low cost.
In recent years, the use of polycrystalline silicon has been investigated to provide as low a cost as amorphous silicon and as high an energy conversion efficiency as single-crystal silicon. In the methods proposed so far, however, a bulk of polycrystalline silicon which is easily prepared is sliced to obtain a plate. It is therefore difficult to make the thickness less than 0.3 mm and keep the electrical resistance low when a photoelectromotive force is obtained. Further, since a plate sliced from a bulk of polycrystal is, for example, polished precisely for use as a photovoltaic device, the plate must have some mechanical strength. Therefore, a minimum thickness is required to enable sufficient light absorption and efficient utilization of material.
To form a photovoltaic device having good efficiency, the semiconductor layer thereof generating photocarriers by light irradiation should preferably be sufficiently thick for light absorption but, at the same time, thickness should be minimized for low device resistance and effective utilization of material. That is, sufficient minimization in thickness is needed for high efficiency and low production cost.
In view of the above, attempts to form a polycrystalline thin film using a thin film forming technique such as the chemical vapor deposition (CVD) method have been tried, but the crystal grain size has been at most several hundredths of a micron and the energy conversion efficiency has been low even compared to the bulk polycrystalline silicon slicing method.
Also, attempts to enlarge crystal grain size by laser light irradiation of a polycrystalline silicon thin film formed according to the above CVD method to cause melting and recrystallization have been tried, but low cost formation has not been accomplished, and stable production has been difficult.
These circumstances are present not only in case of silicon but also in cases of compound semiconductors.
Accordingly, the present applicant provided a thin type of solar cell having a sufficiently large grain size and good energy conversion efficiency, in the Japanese Patent Kokai Gazette No. 63-182872. Described therein is a solar cell comprising a substantially single-crystalline layer of a first conductivity-type semiconductor formed on a substrate based on a foreign material which exhibits a sufficiently larger nucleation density than that the material of the substrate surface and has a sufficiently small area so as to form only a single nucleus from which a single crystal is grown, and a substantially single-crystalline layer of a second conductivity type semiconductor.
FIG. 1 is a schematic drawing of such a solar cell as described in the above-mentioned gazette in which a substrate 11, foreign materials 12, p-type single-crystalline layers 13, i-type single-crystalline layers 14 and n-type single-crystalline layers 15 are found.
This solar cell is prepared using the selective single-crystal growth method. The selective single-crystal growth method causes selective crystal growth on a substrate by utilizing the difference between materials in parameters affecting nucleation in the thin film forming process such as surface energy, attaching factor, detaching factor surface diffusion rate, etc. The method involves growing a single-crystal based on a nucleation surface having a sufficiently larger nucleation density than a nonnucleation surface (having a small nucleation density) on which the nucleation surface is provided and having a sufficiently small area so as to form only a single nucleus from which a single crystal is grown. In this method, no crystal growth occurs from the nonnucleation surface, and a single-crystal is grown only from the nucleation surface.